Devices that sense differential changes in pressure are well known and are commercially available. However they are not without their drawbacks and disadvantages. First, they are typically relatively expensive and are not usually capable of responding with great sensitivity and accuracy, especially with respect to low pressures. Put simply, there are no pressure transmitters commercially available today that are relatively inexpensive and which have the capacity to measure full scale on the order of 0.0015 inch of water column. To even approach this range of sensitivity with any reasonable degree of accuracy results in the instrument being both large and expensive. This is because force multipliers and pressure transmitters of the past that are capable of producing an electrical output have been designed around diaphragms, fulcrums, and levers. Besides the obvious disadvantages associated with size, this design approach gives rise to other problems as well.
With conventional fulcrum and lever design, one has moving points, friction, and slop in the connecting linkage. All of this gives rise to hysteresis problems which contribute greatly to inaccuracies of the force multiplier.
Added to this is the basic problem that comes about due to the diaphragm itself being the measuring element. For very small movements of the diaphragm, there is obviously an output. Yet the very small movements of the diaphragm may be design related or internally generated, and not in any way be a result of a force of pressure change acting on the diaphragm of the instrument. The result of this is a most inaccurate instrument that cannot be relied upon and which has poor repeatability.
My prior invention, as disclosed in U.S. Pat. No. 4,928,529 entitled "Force Multiplying Membrane Instrument" discloses an apparatus that senses an input pressure and produces an electrical output signal that is proportional to the sensed input pressures and satisfactorily overcomes the above noted deficiencies. The apparatus employs a substantially constant area membrane that is preshaped to assume a concave-convex shape and is disposed within a housing structure where it extends across an open chamber within the housing structure and is secured about its periphery by the side walls of the housing structure. For positive multiplication, a force sensing system is disposed on the concave side of the membrane and actually extends into engagement with the membrane for supporting the same about the concave side. Because the membrane is preshaped, it extends inwardly at a selected angle relative to the horizontal plane of the periphery of the membrane secured within the housing. The force applied to the convex side of the membrane and transferred against the force sensing system is multiplied because of the angle of the membrane and the position of the force sensing system.
While my prior force multiplying instrument (described above) functioned well for pressures in the range 0.25 inch of water column, resolution faded below that level due to slight side loading which created slight curvilinear movement of the membrane which limited sensitivity and degraded accuracy.
In view of the foregoing it is therefore an object of the present invention to provide an improved pressure transmitter that senses pressures in the range of 0.00015 inch of water column.
It is a further object of the present invention to provide a pressure transmitter that produces an electrical output signal that is proportional to the applied input pressure.
Another object of the present invention is to provide a pressure transmitter that is relatively small and inexpensive to produce.
Yet another object of the present invention is to provide a pressure transmitter that remains accurate regardless of spatial orientation and that substantially eliminates side loading.
A still further object of the present invention is to provide a pressure transmitter wherein the electrical output signal does not drift over time.
Still another object of the present invention is to provide a pressure transmitter that overcomes the inherent disadvantages associated with like instruments.